Disclaimer: This is totally unsupported! I won't be responsible if you Mac Pro catches fire or simply refuses to boot afterwards. I did the update on a 2006 Mac Pro, running the MP21.007F.B06 EFI firmware and 1.7f10 SMC firmware. Anything else, I can't guarantee.

Following MacEFIRom's work on his Mac Pro 2006-2007 Firmware Tool (you need to be registered to see the download link), here is a way to update the SMC, to complete the 2006-2007 conversion.

The first 5 bytes replace the call to the function that checks whether the upgrade path is approved by Apple (isValidConfig) by xor eax, eax; nop; nop; nop.The next 9 bytes replace a comparison and a conditional jump (related to a global variable set by isValidConfig) by 9 nops.And the last 5 bytes replace the call to the function that prevents from downgrading the SMC firmware by xor eax, eax; inc eax; nop; nop.

If you edited the file properly, its md5 sum should now be 84dbe9708eafc0c29653414b06292f8e.

In order to use it, copy the two files to a EFI accessible partition (FAT or HFS), boot to the EFI Shell, and simply issue the command:

SmcFlasher.efi -LoadApp m43a.smc

To boot the EFI Shell, simply install rEFIt. you can also take a shell.efi binary (for instance, from rEFIt's tools), rename it to boot.efi and copy it at the root of a FAT formatted USB key. You can then boot the key using the Mac Pro built-in boot selector (holding Alt before the chime).